1. Field of the Invention
The present invention relates to a zoom lens. More particularly, the invention relates to a zoom lens which is structured to arrange its entrance pupil further on the object side than the first plane of the lens on the object side such as a zoom tube lens for a microscope.
2. Related Background Art
Traditionally, there have been known for a zoom tube lens for a microscope of the kind, a zoom lens of a four-group construction having positive, negative, negative, and positive refracting powers sequentially from the object side and a zoom lens of a three-group construction having positive, negative, and positive refracting powers as the embodiments disclosed in Japanese Patent Publication No. 2-54925, for example. The above-mentioned zoom lenses are both of such a structure that zooming is performed by driving the second lens group and third lens group.
Also, there is known a zoom lens of a four-group construction having positive, negative, positive, and positive powers, which is widely employed for a phototaking zoom lens as disclosed in Japanese Patent Laid-Open Application No. 2-66509, for example. This zoom lens is of such a structure that the first, third, and fourth lens groups are driven together toward the object side when zooming is performed from the wide angle side to the telescopic side.
In the conventional zoom lenses disclosed in the Japanese Patent Publication No. 2-54925 and Japanese Patent Laid-Open Application No. 2-66509 as mentioned above, the exit pupil of these zoom lenses is all varied by zooming if the entrance pupil is positioned further on the object side than the plane of the first lens on the object side. Consequently, when this type of zoom lens is used as a zoom tube lens for a microscope, there is a disadvantage that the position of the eye point of an eyepiece lens is varied by zooming. Also, if a relaying optical system should be inserted behind a zoom tube lens for this use, the entrance pupil for the incident light for the relaying optical system is varied by zooming. Thus, there is a disadvantage that the structure of the relaying optical system becomes complicated.